Particle beam, in particular ionic optic imaging system

ABSTRACT

A particle beam, in particular in ionic on the reproduction system, preferably for lithographic purposes, has a particle source, in particular an ion source for reproducing on a wafer a structure designed in a masking foil as one or several transparent spots, in particular openings, through at least two electrostatic lenses arranged upstream of the wafer. One of the lenses is a grating lens constituted by one or two tubular electrodes and by a perforated plate arranged in the path of the beam perpendicularly to the optical axis. The plate is formed by a masking foil which forms the central or first electrode of the granting lens, in the direction of propagation of the beam.

FIELD OF THE INVENTION

The present invention relates to a particle beam imaging system, and inparticular, an ionic optic imaging system.

BACKGROUND INFORMATION

The lithographic step is especially important amongst the differentsteps which must be carried out to produce semiconductor elements. Eachlithographic step commences with the application of a thin sheet oflight-sensitive (or particle beam sensitive) material, the so-calledphoto-resist or for short "resist" onto a wafer made in particular fromsilicium. A lithographic device then projects the structure present on amask in the form of one or several transparent spots onto the waferprovided with the resist. Where appropriate, optical elements arelocated between the mask and the wafer. The expansion of the projectedstructure of the mask on the wafer is mostly much smaller than thewafer. After the projection process, the wafer is displaced and the samestructure of the mask is projected to a different spot on the wafer.This process of projection and displacement is repeated again and againuntil the entire wafer surface has been covered. By virtue of thesubsequent development of the resist, the desired sample for example inthe form of resist-free spots is obtained on the wafer. The wafer canthen be subjected in further steps to any of the known treatmentprocedures, such as etching, ion implantation or application anddiffusion of doping material. The wafer is checked after these furthersteps, coated again with resist and the entire aforementioned stepsequence repeated approximately 10-20 times, until finally a chessboard-type arrangement of identical microcircuits is produced on thewafer.

Most of the conventional projection lithographic methods employ light toirradiate the resist but the requirement for smaller structures andhigher densities of the components of the microcircuits has lead to anintensive search for other irradiation methods which in their resolutionare not limited as is the case when employing light owing to itsrelatively long wave length. Great efforts have been undertaken to useX-ray beams in lithographic devices, whereas other methods such as forexample the particle beam, in particular ion beam lithography have infact been awarded some but considerably less consideration.

For a particle beam, and in particular ionic optic imaging systems,preferably for lithographic purposes, comprising a particle source, inparticular an ion source, for the purpose of imaging onto the wafer astructure located on a mask in the form of one or a plurality of holesby way of at least two collecting lenses located between the mask and awafer, it has already been proposed in European Patent Application No.93 890 058.6 to design at least one of the collecting lenses as aso-called three electrode grating lens which consists of two tubeelectrodes. Between the electrodes is located a third electrode which isdesigned as a plate comprising a plurality of holes, preferably as agrating, wherein the plate, more specifically the grating, is arrangedperpendicular to the optical axis, so that by virtue of the plate, i.e.the grating, the lens is divided into two regions, wherein it ispreferable to provide different voltages at the three electrodes. One ofthe lens regions of the three electrode grating lens having a grating asthe middle electrode can have a positive refractive power, the secondlens region however can have a negative refractive power, wherein theabsolute value of the refractive power of the lens region having thenegative refractive power (the dispersing region) is lower that therefractive power of the lens region having the positive refractive power(collecting region). In the case of an imaging system of this type, itis possible despite the different absolute amounts of the refractivepowers for the image distortion coefficients to be compensated to agreat extent if the dispersing region comprising corresponding greaterimage distortions of the 3rd order than the collecting region.

The use of a three-electrode grating lens thus renders it possible ineach case to cause one image distortion coefficient to disappear intothe imaging equations of this lens, wherein also the remaining imagedistortion coefficients assume small values in comparison to theelectrostatic lenses described for example from the European PatentApplication Nos. EP-89109553, EP-92890165, and EP-9280181. This isachieved by introducing the plate or rather grating electrode since onlythis type of electrode renders it possible to produce an electrostaticdiverging lens. In addition thereto, in ionic optic lithography systemshaving three electrode grating lenses the resulting distortion is alsoreduced at the site of the minimum distortion. The use of the threeelectrode lenses with grating as the middle electrode moreover rendersit possible to reduce the distance mask-wafer with respect to knownsystems if the distortion values are related to identical image fieldsizes. Finally, the intensity of the sharpness of the imaging system issubstantially increased because the three-electrode lenses with agrating as the middle electrode comprise in each case only extremelysmall image distortions and therefore the compensation of the imagedistortion in the case of the lens being located between the wafer andmask is less critical.

Essentially, this effect can be achieved if the sequence of divergingand collecting lenses is reversed, i.e. if for example the electrodedesigned as a grating forms the first electrode of a divergingtwo-electrode grating lens, followed for example by a collecting lens inthe form of a field lens. The term `field lens` or `immersion lens`refers in this case to an arrangement of two coaxial, rotationallysymmetrical electrodes (e.g, tubular electrodes) which are located ondifferent potentials.

Interference caused by the holes in the plate or rather by the holes inthe grating can be avoided, if on the one hand the grating holes andtheir spacing are maintained extremely small (in the order ofmicrometers) and if the field intensities on both sides of the gratingin the region of the illumination by the ion beams are as identical aspossible.

The problem when using gratings resides in the fact that, owing to thefine holes and cross pieces, the grating must be extremely thin yetcover a relatively large area and therefore is extremely sensitive todamage, so that constant monitoring is required both with respect to themanufacturing process and also during its period of use in the lens. Forthis purpose, the grating must be removed from the machine, wherenecessary cleaned and reinstalled in position, wherein each time owingto its fragility there is the risk of it being damaged or destroyed.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to obviate thisproblem. To this end, the present invention describes design of aparticle beam, in particular ionic optic imaging system, preferably forlithography purposes, having a particle source, in particular an ionsource for imaging onto the wafer a structure located on a masking foilin the form of one or a plurality of transparent spots, morespecifically holes, by way of at least two electrostatic lenses arrangedin the beam direction in front of a wafer, wherein at least one of thelenses is a so-called grating lens formed by one or two tube electrodesand a plate which comprises holes and is arranged in the beam path,wherein the plate is arranged perpendicular with respect to the opticalaxis, that according to the present invention the plate is formed by themasking foil which forms the middle or as seen in the beam direction thefirst electrode of the grating lens. In each case the mask together withthe following electrode as seen in the ion beam direction forms adiverging lens whose image distortions of the 3rd order are compensatedto a minimal residual value by the following collecting lens(es).

By replacing the grating of the conventional design with the mask onlyone of the elements provided requires maximum precision during itsmanufacture and use, namely the mask itself, which even in the case ofall other embodiments is to be manufactured with an equally high amountof precision, however, the other element which is likewise to bemanufactured with a high amount of precision is omitted and the defectswhich cause the inadequacies of these second elements are avoided.

In a further embodiment of the present invention the ionic optic imagingsystem, in which the mask represents the middle electrode of a threeelectrode grating lens, comprises as a further imaging element acollecting lens in the form of a field lens or an asymmetrical singlelens. The asymmetrical single lens in this case is understood to be athree-electrode lens, wherein all electrodes are located on differentpotentials. Asymmetric single lenses are used because they haveconsiderably less image distortion than immersion lenses with equivalentimaging characteristics.

In the three-electrode grating lens, whose middle electrode is now themask, the first region represents practically the illuminating lens forthe mask and the second region represents the diverging lens (negativerefractive power) whose distortion is to a great extent compensated bythe lens following on in the beam path.

In a further embodiment, wherein the mask forms the first electrode of atwo-electrode grating lens, following on from this lens in the beam pathare a field lens and then an asymmetric single lens. In this case theimage distortion of the last lens in front of the wafer is compensatedby the distortion of the diverging lens comprising the mask and by thelens following.

Owing to the fact that the mask is now placed at the site where in theconventional design (European Patent Application No. 93 890 058.6) thegrating was located, the grating plane has now become the object plane.The holes in the mask form the aperture lenses, as did the holes in thegrating in the conventional design. Since, however, the mask is now theobject to be imaged, which will be imaged in the image plane, accordingto the Laws of Optics a small change in the angle of the particlesissuing from the mask holes is to a great extent to be disregarded inthe effect on the imaging since all particles emitted from an objectpoint are collected in one image point.

In a further embodiment of this type of design according to the presentinvention, in which the mask forms the first electrode of atwo-electrode grating lens, the cross-over point of the beams("cross-over") lies in the field-free space in the beam direction infront of the last lens before the wafer, i.e between the field lens andthe asymmetric single lens. It is possible in a further embodiment ofthe present invention to provide that the field lens accelerates theions to an energy which is sufficiently high that the so-calledstochastic space charge effect, as described herein, are minimized. Inthe asymmetric single lens the energy of the ions can if necessary bereduced to the energy required at the wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a schematic and lateral view of the structure and beampath of an ion lithography system of a first embodiment with a crossoverpoint, with distortion correction, according to the present invention.

FIG. 1b shows the analogue view of the first embodiment according to thepresent invention without a cross-over point.

FIG. 2 shows an ion lithography system of a second embodiment of thepresent invention with distortion correction.

FIG. 3a shows ion lithography systems of the type as shown in FIGS. 1a,1b and 2 with a correction of the chromatic defect.

FIG. 3b shows another embodiment of ion lithography systems of the typeas shown in FIGS. 1a, 1b and 2 with a correction of the chromaticdefect.

FIG. 4 shows a lens according to the present invention comprising threeelectrodes in the axial longitudinal sectional view, wherein the middleelectrode is formed by a mask according to the present invention.

FIG. 5 shows a combination of a collecting lens as shown in FIG. 4 inthe axial longitudinal sectional view with a further lens in the form ofa field lens arranged in front of the wafer.

FIG. 6 shows a simplified view of an ion projection device according tothe present invention having a two electrode grating lens partially inan axial longitudinal sectional view in which the mask forms the gratingelectrode.

FIG. 7 shows the progression of the chromatic resolution distortion andthe distortion in dependence upon the wafer position for an arrangementaccording to FIG. 6.

FIG. 8 shows a view analogous with that of FIG. 1b, with correcting lensin front of the wafer.

DETAILED DESCRIPTION OF THE INVENTION

According to FIGS. 1a and 1b the ionic optic imaging system comprises anion source Q for the purpose of imaging a structure located on a mask Monto a wafer W. The structure is present as at least one hole on themask M. In the first embodiment according to the present invention, twoof the electrodes, as illustrated in more detail in FIG. 4 and FIG. 5,as tube electrodes R1 and R2 and the third electrode arranged betweenthese tube electrodes R1 and R2 is formed by the mask M. The mask M isarranged perpendicular to the optical axis D of the imaging system anddivides the lens L1 into two regions P and N. Different potentials U1,U2 and U3 lie at the three electrodes, namely the two tube electrodes R1and R2 and the electrode formed by the mask M, the desired beam isdesignated 1 in the drawing and the actual beam is designated 2.

In the example described, the lens region P has a positive refractivepower and the lens region N a negative refractive power, wherein howeverthe absolute amount of the refractive power of the lens region N havingthe negative refractive power is less than the refractive power of thelens region P having the positive refractive power so that the totalrefractive power of the three-electrode lens G3 is positive.

It is further evident from FIG. 4 and FIG. 5, that the tube electrode R2of the lens region N having the negative refractive power has a smallerdiameter (approximately half the size) than the part facing the mask ofthe tube electrode R1 of the lens region P having the positiverefractive power. The voltage ratio (U3-U0)/(U2-U0) between theelectrodes of the lens region N having the negative refractive power islikewise smaller (approximately one third) than the voltage ratio(U3-U0)/U2-U0) between the electrodes of the lens region P having thepositive refractive power, wherein U0 is the potential at which thekinetic energy of the charged particles employed would be zero.

Endeavours are made to provide at both sides of the mask M fieldintensities which are as identical as possible for each region which isilluminated by the beam of ion rays.

In the three-electrode grating lens which now comprises the mask as thegrating, the first region represents practically the illuminating lensfor the mask and the second region represents the diverging lens(negative refractive power) whose distortion is to a great extentcompensated by the following collecting lens L2. Owing to the fact thatthe mask is now located at the site where in the conventional design(European Patent Application No. 93 890 058.6) the grating was located,the grating plane now becomes the object plane. The holes in the maskform practically the holes of the grating, wherein the mask holes havethe same effect as the aperture lenses of the grating holes. Since,however, as already mentioned, the mask M is now the object to be imagedin the optical column, which object will be imaged in the image plane,according to the Laws of Optics a small change in the angle of theparticles issuing from the mask holes is to a great extent to bedisregarded in the effect on the imaging since all particles emittedfrom an object point are collected in one image point.

The explanations apply both for the design of the second lens L2 as anasymmetric single lens and also as a field lens.

The mask M which comprises the structure to be imaged in the form ofholes in a foil (e.g. made from silicium) is illuminated by an ionsource Q having an extremely small virtual source size (almost 10 μm)and following thereon illuminating device, which can comprise twomultipoles and a mass separator (e.g, ExB filter) disposed between thetwo multipoles, and where appropriate a diagnosis system. The said maskin a first embodiment according to the present invention, is locatedbetween the outer electrodes of the lens G3 consisting of threeelectrodes R1, R2 and M and having an overall positive refractive power.

In the embodiment of the present invention, as illustrated in FIG. 1a,the lens G3 produces a cross-over region (crossover) C, i.e. a realimage of the ion source Q behind its image-side focal point. Theobject-side focal point of the second collecting lens L2 arranged infront of the wafer W is located approximately at the site of thecross-over C. Thus, all beams leave the collecting lens L2 almost axisparallel and an almost telecentric imaging system is obtained. This hasthe advantage that the imaging scale in the case of small displacementsof the wafer W does not change in the direction of the ionic optic axis.

In another embodiment of the present invention, as shown in FIG. 1b, animage of the mask can also be produced without a cross-over point. Inthe case of an image reducing in size the telecentricity is lost,wherein extremely small angles (approx. 6 mrad) are still possible atthe wafer. In order to regain a telecentric system it is possible inaccordance with the present invention to provide a correcting lens WLdirectly in front of the wafer (as shown in FIG. 8), by means of whichthe beams impact on the wafer again in an almost axis parallel manner.According to the present invention, this correcting lens can be formedin such a manner that a single substantially tubular electrode ispositioned directly in front of the wafer and in comparison therewithcomprises a slightly different potential. This electrode forms togetherwith the wafer the so-called wafer lens WL which again comprises aplate, namely the wafer itself, as the electrode and can therefore beoperated as a diverging lens. This is then the case when the potentialat the electrode lying in front of the wafer is such that the ions aredecelerated on the way to the wafer.

In a grating design of the three-electrode lens with grating accordingto the present invention the distance between the facing ends of thetube electrodes R1, R2 amounts to 135 mm. The electrode formed as themask M is arranged at a distance of 90 mm from the outlet port of thetube electrode R1, wherein the diameter of the outlet port amounts to600 mm, whereas the diameter of the inlet port of the tube electrode R2amounts to 300 mm. The outlet port of the tube electrode R2 is 675 mmaway from the electrode formed as the mask M and the inlet port of thetube electrode R1 lies 1350 mm in front of the mask forming the middleelectrode. At a distance of 585 mm from the outlet port facing the maskM, the inner diameter of the tube electrode R1 reduces from 600 to 300mm.

In the case of another embodiment of the present invention asillustrated in FIG. 6 the mask M forms the first electrode of thediverging grating lens G2. There then follows the collecting lens L1,followed by the second collecting lens L2 which lies in front of thewafer plane in the beam path and is formed as an asymmetrical SINGLElens. Since the potential of the last electrode of a lens is always thesame as that of the first electrode of the following lens, these twoelectrodes can be formed physically in each case as a single electrode,as can also be seen in FIG. 6. It is therefore possible to regard theentire ionic optic column between the mask and wafer as a single"multi-electrode lens", in this case as a five electrode lens, whichcomprises different refractive regions, namely G2, L1 and L2. Whereappropriate, for example in order to reduce the distance between thesource and the wafer, it is also possible to provide an illuminatinglens in front of the mask not illustrated in FIG. 6!.

In contrast to the previously described three-electrode lens, in whichthe mask M forms the middle electrode, in the case of the arrangement asshown in FIG. 6 the field intensities in front of and after the mask aredifferent, so that mini-lens effects (flies eyes lens effects) occurcaused by the holes in the mask M. Since, however, the electric fieldintensity at the plane of mask M which faces the second electrode isextremely low and because the object being imaged is the mask, then theeffect of this mini-lens is negligibly small. The low field intensity isdesired since owing to the work which must be provided by a foil movingin an electric field, any occurring vibrations of the mask M areavoided. This renders it possible to change and set up the mask Mrapidly. The arrangement as shown in FIG. 6 renders it possible to throwthe image of mask holes with a telecentricity better than 2 mrad ontothe plane of the wafer W. This high telecentricity produces in the planeof the wafer W a sharpness intensity of approximately 10 μm. Thereforein this embodiment of the present invention, it is not only the imagingscale and the resolution which is maintained, even when the waferchanges position in the optical axis, but it is also possible tomaintain the particularly small distortion.

In the case of the system as shown in FIG. 6 the ion beam impacts in a60×60 mm² mask structure field at angles which are less than 1.5° at theedge of the mask field. The mask holes can be achieved in a 3 μm thicksilicium foil having a bias for 3°. It is thus possible in a 3 μm thickmasking foil to achieve line gratings with a hole width of e.g. 0.45 μmwithout shadow distortion occurring caused by the mask edges. Thedistance between the wafer W and the mask M amounts for example to2171.51 mm and the maximum diameter of 1180 mm.

The devices according to the present invention comprise irrespective ofwhich embodiment the following characteristics:

a) In the case of the arrangements comprising a cross-over the beam ofrays generally experience a cushion-shaped residual distortion (regionA) by virtue of the collecting lens G3N (FIGS. 1a and 1b) and G2 and L1(FIG. 2) which follow and/or contain the mask M, which cushion-shapedresidual distortion changes after the cross-over to become abarrel-shaped residual distortion (Region B).

FIGS. 1a and 2 illustrate that by virtue of the distorting effect of thecollecting lens L2 disposed in front of the wafer W the beams experiencean additional deflection which reduces the region B of a barrel-shapeddistortion and generates a region A' of cushion-shaped distortion whichis adjacent to the region of a barrel-shaped distortion. Referring toFIG. 1b, in the case of the arrangement without a cross-over thecushion-shaped distortion generated by the collecting lens G3N isreduced by virtue of the lens L2 and finally becomes a barrel-shapeddistortion (Region B).

A plane is produced behind the collecting lens L2 arranged in front ofthe wafer W, in which plane the distortions caused by the lenses lyingbetween the mask and the wafer are compensated (plane of minimaldistortion). This, however, only applies for the distortion of the 3rdorder, the much smaller distortions of the 5th and 6th order remain, sothat the distortion does not completely disappear but rather is merelykept to a significant minimum. When using a wafer lens WL, as describedbelow, this can contribute to a further minimizing of the distortion.

The dependence of the residual distortion on the deviation in the beamdirection of the wafer position from the desired position in theproposed embodiment of the present invention according to FIG. 6progress with practically the same increase as in the device describedin the European Patent Application No. 93 890 058.6, in which two threeelectrode grating lenses for the purpose of imaging the holes of, a maskare used (FIG. 7), wherein the distortion minimum is less than in theaforementioned application. Together with the higher telecentricity thisleads to a sharpness intensity of approximately 10 μm which issubstantially better than the sharpness intensity achievable in aconventional imaging process using light optics. For example, in thecase of photolithography using high energy UV light ("deep UV") duringthe manufacture of structures of 0.2 μm the sharpness intensity islikewise in the region of only some tenths μm. In order to be able toproduce integrated circuits with these low sharpness intensities, it isnecessary in the case of light optics to level the surface of the waferand the lithography may only take place in the uppermost layer ("topsurface imaging"), which is associated with high costs.

b) In the arrangements as shown in FIGS. 1b and 2, as is evidentschematically from FIG. 3a and 3b, a compensation for the chromaticdefect is provided by virtue of the effects of the two collecting lensesL1 and L2 for a predetermined image plane E after the second collectinglens arranged in front of the wafer W. A beam (illustrated in FIGS. 3aand 3b with a broken line) having an approximately smaller energy E_(o)-ΔE_(o) than the desired energy E_(o) is deflected in the lens G3 andG2+L1, which contains the mask M as an electrode, to a greater extentthan a beam having the desired energy (illustrated by the continuousline) and as a result thereof meets the lens L2 arranged in front of thewafer W at a greater distance to the axis than the desired beam and istherefore owing to its lower energy deflected back to a greater extentwith respect to the optical axis, so that it meets the desired beam in apredetermined distance behind the collecting lens L2 arranged in frontof the wafer W. In this plane E the chromatic defect of the 1st orderdisappears. A residual distortion, the chromatic defect of the 2nd orderalso remains here, i.e. a distortion proportional to the square of theenergy deviation from the desired beam.

In the case of the arrangement as shown in FIG. 1a the wafer lens WL (asshown in FIG. 8) can be used to minimize the chromatic defect.

The three relevant planes, namely the Gaussian image plane of the maskM, the plane of minimal distortion (FIGS. 1a and 1b and 2) and the planeof minimal chromatic defect (FIG. 3a and FIG. 3b) generally do notcoincide. By appropriately selecting the lenses and the position of theion source Q on the ionic optic axis it is, however, possible to achievethat while maintaining the almost parallel beam path the said threeplanes coincide at the outlet of the lens L2. If then the wafer W isarranged where the three planes coincide, then this has the effect thatboth the distortion and also the chromatic defect are minimal.

In the case of all the described embodiments comprising a cross-over(FIG. 1a, FIG. 2, FIG. 6) a highest possible particle energy is achievedat the cross-over, which has the advantage that the so-called stochasticspace charge effects are minimized. The cross-over lies in all cases inthe field free space in front of the lens L2.

Stochastic particle deflections are for example discussed in the article"Stochastic ray deflections in focused particle beams" by A.Weidenhausen, Speer and H.Rose in Optik 69 (1985, pages 126 to 134. Thestochastic space charge effect leads to resolution defects during theimaging process, i.e. to a reduction of the maximum achievableresolution of the ionic optic. The intensity of this effect dependsamong other things on the energy at the cross-over and in fact accordingto the above article as in accordance with:

    δX˜E.sup.5/4

wherein δX represents the deterioration in the resolution and Erepresents the energy of the ions at the cross-over.

The following table shows the calculation results for the embodiment ofan ion projection lithography system in accordance with the presentinvention as illustrated in FIG. 6.

                  TABLE 1                                                         ______________________________________                                        Distance Source-mask  1.6     mi                                              Distance Mask-wafer   2.2     m                                               Image field size on the mask                                                                        60 × 60                                                                         mm.sup.2                                        Imaging scale (reduction)                                                                           3:1                                                     Voltage ratio at G2   1.082                                                   Voltage ratio at L1   18.16                                                   Voltage ratio between the first                                                                     0.75                                                    and third electrode from L2                                                   Voltage ratio between the first                                                                     0.1825                                                  and second electrode from L2                                                  Maximum distortion*   <10     nm                                              max. chromatic spread 50      nm**                                                                  20      nm***                                           max. beam divergence after the                                                                      <2      mrad                                            lens system                                                                   ______________________________________                                         *Max. deviation of any image point from the ideal image                       **for an energy blurring of ions at the ion source outlet E/E = 0.0003,       wherein E represents the energy of the ions at the outlet of the ion          source.                                                                       ***for ion sources, where the ions at the outlet comprise less energy         blurring, namely E/E = 0.0001.                                           

Under the above term voltage ratio between two electrodes of one lenswith the potentials U₁ and U₂, the following ratio is to be understood:##EQU1## wherein U₀ is the potential at which the kinetic energy of thecharged particles would be equal to zero.

What is claimed is:
 1. A particle beam imaging system, comprising:amasking foil; a particle source projecting a representation of astructure onto a wafer along a beam path, the structure being formed onthe masking foil and having at least one transparent portion; and atleast two electrostatic lenses having an optical axis therethrough andbeing positioned between the wafer and the particle source, a first lensof the at least two electrostatic lenses including a grating lens formedby a first tube electrode and a first plate, the first plate beingpositioned perpendicular to the optical axis, wherein the masking foilforms the first plate, and wherein the masking foil is positioneddownstream of the first electrode in a direction of the beam path. 2.The particle beam imaging system according to claim 1, wherein theparticle source is an ion source.
 3. The particle beam imaging systemaccording to claim 1, wherein the at least one transparent portionincludes a plurality of holes.
 4. The particle beam imaging systemaccording to claim 1, wherein the grating lens further includes a secondtube electrode.
 5. The particle beam imaging system according to claim1, further comprising:an additional lens positioned in the beam pathbetween the masking foil and the wafer, the additional lens including athird tube electrode and a second plate, wherein the wafer forms thesecond plate and wherein the third tube electrode is positioned directlyin front of the wafer.
 6. The particle beam imaging system according toclaim 1, wherein the at least two electrostatic lenses and the maskingfoil form a three-electrode grating lens, the masking foil being amiddle electrode of the three-electrode grating lens, and wherein asecond lens of the at least two electrostatic lenses is an immersionlens formed between the masking foil and the wafer.
 7. The particle beamimaging system according to claim 1, wherein the at least twoelectrostatic lenses and the masking foil form a three-electrode gratinglens, the masking foil being a middle electrode of the three-electrodegrating lens, and wherein a second lens of the at least twoelectrostatic lenses is an asymmetric single lens, the asymmetric singlelens being formed between the masking foil and the wafer.
 8. Theparticle beam imaging system according to claim 1, wherein the gratinglens is a two-electrode grating lens, the masking foil being the firstelectrode of the two-electrode grating lens, the second lens beingformed as a two electrode immersion lens and positioned after thetwo-electrode grating lens, and further comprising:a third lens formedas an asymmetric single lens, the third lens being positioned betweenthe second lens and the wafer.
 9. The particle beam imaging systemaccording to claim 1, wherein the particle source projects the structureusing a plurality of ion beams, a cross-over point of the plurality ofion beams being positioned in a field-free space.
 10. The particle beamimaging system according to claim 9, wherein the plurality of ion beamsgenerate a first energy at the cross-over point and a second energy at athe wafer, the first energy being greater than the second energy. 11.The imaging system according to claim 1, wherein the beam path betweenthe mask and the wafer excludes a cross-over point.